Ok.

My intention of comparing UML with XML, was not to draw any technology similarities between these two technologies. I think that, UML and XML are unrelated technologies.

I guess that, to model complex OO applications, many people instead of using UML, use things like free form diagrams, flow charts etc. The disadvantage of that many times is, that, people invent their own notational conventions (like, what an arrow within a diagram means) for a particular task (which they often explain, with supplementary documentation). For that reason, its useful to use notations like UML to model complex OO applications, because that is an OMG standard.